The invention relates to the output stage employed in a monolithic silicon integrated circuit (IC) amplifier. One well-known form of output stage is disclosed and claimed in U.S. Pat. No. 3,974,456, which was issued to Ronald W. Russell and Kyle M. Black, on Aug. 10, 1976. This patent, which is assigned to the assignee of this application, teaches a pair of NPN output transistors which are biased by means of P-channel junction field effect transistor (JFET) devices. This circuit is exemplified in the LF156 which is available commercially from National Semiconductor Corporation and others.
Another commonly used IC amplifier output stage design which employs a complementary transistor is exemplified in the LM163, which is also commercially available from National Semiconductor Corporation and others. Here, an NPN transistor acts as an output terminal current source and hence provides the pull up function. A PNP transistor acts as a current sink to provide the pull down function. This configuration provides excellent performance but suffers an asymmetry in its current sinking capability. The PNP transistor is not able to sink as much current as the NPN transistor can source. It also suffers to a small degree, at the higher frequencies, from an asymmetry in the gain versus frequency roll off. In general, IC PNP transistors have a much lower gain bandwidth than the NPN transistors. When the complementary device circuit is employed at the higher frequencies, the output will display a phase shift between the positive and negative signal swings.
From the above, it is clear that NPN output devices are preferred, but, since the most popular such circuit employs JFET biasing, the JFET characteristics must be maintained under stringent manufacturing control.